1. Field of the Invention
The invention is directed to an ion chamber type neutron detector and particularly such detectors used to measure the neutron flux in a nuclear reactor core.
2. Description Of The Prior Art
An example of an in-core neutron detector system of the type which the present invention may be employed is shown by G. R. Parkos et al. in U.S. Pat. No. 3,565,760.
Ion chamber type neutron detectors are well known and are shown, for example, by L. R. Boyd et al. in U.S. Pat. No. 3,043,954. Usually such chambers comprise a pair of spaced electrodes electrically insulated from one another with a neutron sensitive material and an ionizable gas therebetween. For example, in a fission type ion chamber the neutron sensitive material is a material such as uranium which is fissionable by neutrons. As neutrons induce fissions of the uranium in the chamber, the resultant fission products ionize the gas in proportion to the magnitude of the neutron flux in the chamber. When a direct current voltage is applied across the electrodes, an output current is created which is proportional to the amount of ionization and hence proportional to the neutron flux in the chamber.
An inherent and serious difficulty with presently known fission type ion chamber neutron detectors is their relatively limited lifespan due to depletion of the fissionable or active material therein. For example, in-core fission chamber neutron detectors presently being used have a lifespan of approximately 1.4 to 2 years and newer more advanced reactor core configurations having higher neutron flues are now being planned that could create conditions lowering neutron detector lifespan to approximately one year. Depletion of the active material in the neutron detector necessitates costly and time-consuming periodic replacement of the detectors.
Detector life cannot be lengthened merely by increasing the initial amount of active material in the detector. The amount of active material that can be used in the detector is limited by several factors including the need for a small active gas volume to minimize sensitivity to gamma radiation and the requirement that the coating of active material be sufficiently thin to allow escape of fission products into the active gas volume for contribution to the ionization process.
It has been recognized for some time that detector lifespan can be extended by combining with an initially active fissionable material a breeding material such as U-234, U-238, Pu-238, Pu-240 and Th-232 which upon capture of neutrons is converted to a fissionable isotope to thereby continuously replenish the active material of the detector. Such regenerative detectors have been suggested, for example, by D. E. Hegberg in a paper entitled "Feasibility Study of In-Core Neutron Flux Monitoring With Regenerating Detectors." HW-73335 Hanford Laboratories, June 1972 and O'Boyle et al. U.S. Pat. No. 3,742,274.
It is also known that a strong factor affecting the lifespan of an in-core detector is the magnitude of neutron flux in the region in which the detector is located in the core. Assuming sufficient detector sensitivity, detector lifespan can also be extended by reducing the magnitude of neutron flux to which detector is exposed. In the prior art this has been accomplished by reducing the amount of neutron moderator in the vicinity of the detector and/or by surrounding the detector by a suitable neutron shielding material or a burnable poison material. However, these prior art methods of extending neutron detector lifespan have not sufficiently extended the lifespan of in-core neutron detectors.
Thus, it is the principal object of the present invention to provide an ion chamber type neutron detector with a usable lifespan far greater than those possible with prior art methods of extending detector lifespan.
It is another object of the invention to improve nuclear reactor operation economics by substantially reducing or eliminating costly and time-consuming periodic replacement of in-core neutron detectors.